i) Field of Invention
Embodiments of the present invention generally relate to motion compensation, and in particular to an autofocus-based compensation (ABC) systems and methods for a ground moving target indication (GMTI) sensor.
ii) Description of Related Art
Various military and non-military organizations use moving target indication (MTI) radar systems—both ground-based and airborne. Most of these systems process data in the frequency domain, detecting the Doppler shift induced on a transmitted waveform by the radial velocity of the moving target. Since they process target returns appearing at specific Doppler frequencies, it is important that their relative spatial location be precisely specified throughout the entire processing interval. Without accurate position information, processing algorithms cannot correct for phase errors introduced by platform motion. This is generally not an issue for stationary, ground-based systems; but it is critically important for airborne systems, including the helicopter-based multi-channel system that was considered.
In a typical hovering airborne GMTI radar system, a post-processor uses simultaneously collected platform position and orientation information to correct for motion-induced errors in the measured signal phases. This motion compensation (MOCOMP) system performs well provided that the position measurement system produces high-precision outputs. If, however, the position measurements are imprecise; then the downstream GMTI algorithm may be unable to detect specific targets of interest. This may be especially true when the target of interest has low radar cross section (RCS) and is moving at low speed. The small amount of energy available in its GMTI signature would likely smear across multiple Doppler bins, making it indistinguishable from large sidelobes of the main clutter ridge concentrated near the 0 Hz Doppler bin.